The present invention relates to a system for generating percussive sounds in an electronic organ, such as those produced by a piano, harpsichord, banjo and the like. More particularly, the invention is concerned with a percussion generator of this type which is responsive to a multiplexed serial data stream.
Percussive instruments, such as pianos and harpsichords, produce a sound which has a relatively fast attack and a sustain of limited duration. In the case of a piano, for example, when one of the keys is depressed, the sound rapidly attacks to the maximum, decays rapidly to a sustain level and then sustains gradually as the key continues to be held. In a harpsichord, the sustain is much shorter because the strings are plucked rather than struck. A characteristic of all percussive instruments of this type is that the sustain is of limited duration, and even though the key may be held depressed for a long period of time, the sound will decay completely in a relatively short interval.
In electronic organs, the generation of tones is controlled by switches that are actuated by depression of the keys of the keyboard, and for most voicing, as long as the key is held depressed the tone will continue at substantially the same amplitude until the key is released. In order to simulate percussive sounds, then, it is necessary to provide for an attack and decay envelope that is limited in duration, regardless of whether or not the key continues to be held, and yet produce the tone again if the key is released and again depressed. Prior electronic organs employed resistor-capacitor circuits for supplying the keyers with an envelope voltage which is applied only momentarily to simulate the percussive effect, even though the keys are held for a longer period of time than that necessary to produce the percussive attack and decay. A significant disadvantage to utilizing this technique is that a separate percussion circuit must be provided for each of the keyers, and the large number of keyers which must be supplied results in a substantial additional cost if percussion effects are to be provided for all of the tones capable of being played by the organ. Additionally, there is the problem of matching the elements of the resistor-capacitor percussion circuits so that they will produce uniform percussive effects.
For some time now, the keyboards of electronic organs have been multiplexed in an attempt to reduce the number of wiring connections between the keyboard and the keyers associated with it. In an organ of this type, the keys of the keyboard are sequentially scanned and a time division multiplexed serial data stream is produced wherein time slots in the data stream correspond to respective keys of the keyboard and a pulse or other keydown signal will appear in that time slot corresponding to a depressed key. In many multiplexed organs, however, it was still necessary to utilize discrete RC percussion circuits for each of the keyers in order to produce the percussion sounds.
U.S. Pat. No. 4,012,982 provides one solution to the problem of economically providing for percussion in a multiplexed organ. In this system, a delay circuit comprising a shift register having a delay period equal to an integer multiple of the number of time slots in the multiplexed data stream delays the data stream and then recombines it with the undelayed data stream in a gate. The output of the gate is a time division multiplexed signal with pulses in time slots corresponding to the time slots of the multiplexed signal applied to the shift register, but from which pulses are deleted after one or more scans of the manual. The net result is that a burst of keydown pulses are produced for a time period equal to the delay provided by the shift register. Although this system is effective in greatly reducing the cost of providing percussion effects, it has a disadvantage in that the duration of the burst cannot easily be controlled, since it is directly proportional to the length of the shift register delay. Additionally, if longer bursts are desired, a shift register of excessively long length is required, thereby adding significantly to the cost of the percussion circuit.
One disadvantage to generating percussion by means of a burst of keydown pulses over a fixed time interval is not being able to provide a snub effect, which is the rapid decay produced when a piano key, for example, is released before the sustain has decayed out. In a multiplex percussion generator, the burst of tones is applied to regular sustain keyers, which produce the type of attack and decay which are desired to simulate the instrument in question. Since the tone production is generally independent of whether or not the key is released or remains depressed, however, the sustain characteristics will be the same for all notes, regardless of how they are played on the keyboard.
A further disadvantage to producing percussion through a burst of keydown pulses over a fixed interval is the overlap of the sustains of a large number of the notes if the notes are played very rapidly in succession. For example, if a glissando is played by rapidly drawing the finger across the keyboard so that the notes are played in rapid succession, before one note decays out, the next note begins to attack so that very quickly the output circuitry is loaded with tones of different pitches and decaying out at different times. This problem is particularly troublesome in the percussion generator of the present invention wherein the interval of time during which the burst of keydown pulses is produced is lengthened if two notes are played in such rapid succession that the second note is played before the "delta data" pulse produced each time a new key is depressed has terminated. This results in a substantial increase in the sustain times of each of the notes so that an unnatural and unpleasing glissando effect results.